During meiosis, after DNA replication, the pairing of chromosomes involves four copies of each chromatid, which synapse together before recombination takes place. Certain short guanine-rich motifs in single-stranded DNA self-recognize and associate in physiological salt to make four-stranded structures in which the strands run in a parallel fashion. Such four-stranded structures, named G4-DNA, are able to crosslink four Watson-Crick duplexes. Such guanine-rich sequences have been found in immunoglobulin switch regions and in the telomeres of chromosomes. It is possible that this self-recognition of G-rich motifs in DNA is used to bring together the four chromatids that must synapse in meiosis, where a series of different G4-DNA-forming regions along the entire length of a chromosome might serve as an identification pattern for pairing. G4-DNA crosslinks may also be used to align chromosomes in mitosis and in the formation of polytene chromosomes. This project will test the hypothesis that G4-DNA-forming regions serve as identification signals for chromosome matching. Anti-G4- DNA antibodies will be used to detect G4-DNA in chromosomes during meiotic pairing. Crosslinks between chromosomes involved in meiotic pairing will be isolated to investigate whether their sequences are such that they can form G4-DNA structures. Such G4- DNA-forming elements and their surrounding sequences will be cloned, and the engagement of guanines into G4-DNA in vivo will be demonstrated by in vivo dimethyl sulfate methylation experiments. The cell may control the formation of G4-DNA at special times in the cell cycle by synthesizing G4-DNA-binding proteins and special DNA melting proteins. Both proteins that recognize G4-DNA and melting proteins that relax the DNA structure in specific regions, in which one strand is unusually G-rich, will be sought. By binding the C-rich strand, such melting proteins would free the G- rich regions to seek each other out from chromatid to chromatid to make crosslinks to bring together four duplexes. The significance of the G4-DNA hypothesis is that it provides an understanding of how chromosome pairing is accomplished throughout eukaryotic organisms and explains why four chromatids are involved in recombination.